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Neal, BP, Condit C, Liu G, dos Santos S, Kahru M, Mitchell BG, Kline DI.  2014.  When depth is no refuge: cumulative thermal stress increases with depth in Bocas del Toro, Panama. Coral Reefs. 33:193-205.   10.1007/s00338-013-1081-6   AbstractWebsite

Coral reefs are increasingly affected by high-temperature stress events and associated bleaching. Monitoring and predicting these events have largely utilized sea surface temperature data, due to the convenience of using large-scale remotely sensed satellite measurements. However, coral bleaching has been observed to vary in severity throughout the water column, and variations in coral thermal stress across depths have not yet been well investigated. In this study, in situ water temperature data from 1999 to 2011 from three depths were used to calculate thermal stress on a coral reef in Bahia Almirante, Bocas del Toro, Panama, which was compared to satellite surface temperature data and thermal stress calculations for the same area and time period from the National Oceanic and Atmospheric Administration Coral Reef Watch Satellite Bleaching Alert system. The results show similar total cumulative annual thermal stress for both the surface and depth-stratified data, but with a striking difference in the distribution of that stress among the depth strata during different high-temperature events, with the greatest thermal stress unusually recorded at the deepest measured depth during the most severe bleaching event in 2005. Temperature records indicate that a strong density-driven temperature inversion may have formed in this location in that year, contributing to the persistence and intensity of bleaching disturbance at depth. These results indicate that depth may not provide a stress refuge from high water temperature events in some situations, and in this case, the water properties at depth appear to have contributed to greater coral bleaching at depth compared to near-surface locations. This case study demonstrates the importance of incorporating depth-stratified temperature monitoring and small-scale oceanographic and hydrologic data for understanding and predicting local reef responses to elevated water temperature events.

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Murakami, H, Sasaoka K, Hosoda K, Fukushima H, Toratani M, Frouin R, Mitchell BG, Kahru M, Deschamps PY, Clark D, Flora S, Kishino M, Saitoh S, Asanuma I, Tanaka A, Sasaki H, Yokouchi K, Kiyomoto Y, Saito H, Dupouy C, Siripong A, Matsumura S, Ishizaka J.  2006.  Validation of ADEOS-II GLI ocean color products using in-situ observations. Journal of Oceanography. 62:373-393.   10.1007/s10872-006-0062-6   AbstractWebsite

The Global Imager (GLI) aboard the Advanced Earth Observing Satellite-II (ADEOS-II) made global observations from 2 April 2003 to 24 October 2003. In cooperation with several institutes and scientists, we obtained quality controlled match-ups between GLI products and in-situ data, 116 for chlorophyll-a concentration (CHLA), 249 for normalized water-leaving radiance (nLw) at 443 nm, and 201 for aerosol optical thickness at 865 nm (Tau_865) and Angstrom exponent between 520 and 865 nm (Angstrom). We evaluated the GLI ocean color products and investigated the causes of errors using the match-ups. The median absolute percentage differences (MedPD) between GLI and in-situ data were 14.1-35.7% for nLws at 380-565 nm 52.5-74.8% nLws at 625-680 nm, 47.6% for Tau_865, 46.2% for Angstrom, and 46.6% for CHLA, values that are comparable to the ocean-color products of other sensors. We found that some errors in GLI products are correlated with observational conditions; nLw values were underestimated when nLw at 680 nm was high, CHLA was underestimated in absorptive aerosol conditions, and Tau_865 was overestimated in sunglint regions. The error correlations indicate that we need to improve the retrievals of the optical properties of absorptive aerosols and seawater and sea surface reflection for further applications, including coastal monitoring and the combined use of products from multiple sensors.

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Kahru, M.  1997.  Using satellites to monitor large-scale environmental change: A case study of cyanobacteria blooms in the Baltic Sea. Monitoring algal blooms : new techniques for detecting large-scale environmental change. ( Kahru M, Brown CW, Eds.).:43-61., Berlin ; New York: Springer Abstract
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Stukel, MR, Kahru M, Benitez-Nelson CR, Decima M, Goericke R, Landry MR, Ohman MD.  2015.  Using Lagrangian-based process studies to test satellite algorithms of vertical carbon flux in the eastern North Pacific Ocean. Journal of Geophysical Research-Oceans. 120:7208-7222.   10.1002/2015jc011264   AbstractWebsite

The biological carbon pump is responsible for the transport of similar to 5-20 Pg C yr(-1) from the surface into the deep ocean but its variability is poorly understood due to an incomplete mechanistic understanding of the complex underlying planktonic processes. In fact, algorithms designed to estimate carbon export from satellite products incorporate fundamentally different assumptions about the relationships between plankton biomass, productivity, and export efficiency. To test the alternate formulations of export efficiency in remote-sensing algorithms formulated by Dunne et al. (2005), Laws et al. (2011), Henson et al. (2011), and Siegel et al. (2014), we have compiled in situ measurements (temperature, chlorophyll, primary production, phytoplankton biomass and size structure, grazing rates, net chlorophyll change, and carbon export) made during Lagrangian process studies on seven cruises in the California Current Ecosystem and Costa Rica Dome. A food-web based approach formulated by Siegel et al. (2014) performs as well or better than other empirical formulations, while simultaneously providing reasonable estimates of protozoan and mesozooplankton grazing rates. By tuning the Siegel et al. (2014) algorithm to match in situ grazing rates more accurately, we also obtain better in situ carbon export measurements. Adequate representations of food-web relationships and grazing dynamics are therefore crucial to improving the accuracy of export predictions made from satellite-derived products. Nevertheless, considerable unexplained variance in export remains and must be explored before we can reliably use remote sensing products to assess the impact of climate change on biologically mediated carbon sequestration.

Kahru, M, Elmgren R, Di Lorenzo E, Savchuk O.  2018.  Unexplained interannual oscillations of cyanobacterial blooms in the Baltic Sea. Scientific Reports. 8   10.1038/s41598-018-24829-7   AbstractWebsite

Population oscillations in multi-species or even single species systems are well-known but have rarely been detected at the lower trophic levels in marine systems. Nitrogen fixing cyanobacteria are a major component of the Baltic Sea ecosystem and sometimes form huge surface accumulations covering most of the sea surface. By analysing a satellite-derived 39-year (1979-2017) data archive of surface cyanobacteria concentrations we have found evidence of strikingly regular interannual oscillations in cyanobacteria concentrations in the northern Baltic Sea. These oscillations have a period of similar to 3 years with a high-concentration year generally followed by one or two low-concentration years. Changes in abiotic factors known to influence the growth and survival of cyanobacteria could not provide an explanation for the oscillations. We therefore assume that these oscillations are intrinsic to the marine system, caused by an unknown, probably mainly biological mechanism that may be triggered by a combination of environmental factors. Interactions between different life cycle stages of cyanobacteria as well as between predator-prey or host-parasite are possible candidates for causing the oscillations.

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Schwarz, JN, Kowalczuk P, Kaczmarek S, Cota GF, Mitchell BG, Kahru M, Chavez FP, Cunningham A, McKee D, Gege P, Kishino T, Phinney DA, Raine R.  2002.  Two models for absorption by coloured dissolved organic matter (CDOM). Oceanologia. 44:209-241. AbstractWebsite

The standard exponential model for CDOM absorption has been applied to data from diverse waters. Absorption at 440 nm (a(g)440) ranged between close to zero and 10 m(-1), and the slope of the semilogarithmic absorption spectrum over a minimum range of 400 to 440 nm (s440) ranged between < 0.01 and 0.04 nm(-1). NO relationship was found between a(g)440 or s440 and salinity. Except in the southern Baltic, s440 was found to have abroad distribution (0.0165 +/- 0.0035), suggesting that it should be introduced as an additional variable in bio-optical models when a(g)440 is large. An alternative model for CDOM absorption was applied to available high quality UV-visible absorption spectra from the Wisla river (Poland). This model assumes that, the CDOM absorption spectrum comprises distinct Gaussian absorption bands in the UV, similar to those of benzene. Five bands were fit to the data. The mean central energy of all hands was higher in early summer (E &SIM;7.2, 6.6, 6.4, 6.2 and 5.5 eV or 172, 188, 194, 200 and 226 nm)) than in winter. The higher energy bands were found to decay in both height and width with increasing salinity, while lower energy bands broadened with increasing salinity. 8440 was found to be correlated with shape parameters of the bands centred at 6.4 and 5.5 eV. While the exponential model is convenient for optical modelling and remote sensing applications, these results suggest that the Gaussian model offers a deeper understanding of chemical interactions affecting CDOM molecular structure.

Kahru, M, Kudela RM, Manzano-Sarabia M, Mitchell BG.  2012.  Trends in the surface chlorophyll of the California Current: Merging data from multiple ocean color satellites. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 77-80:89-98. Abstract

Standard remote sensing reflectance products from four ocean color sensors (OCTS, SeaWiFS, MODISA, MERIS) and over 10,000 in situ measurements of surface chlorophyll-a (Chl-a) concentration in the California Current were used to create empirical algorithms that are consistent with in situ data as well as between individual sensors. Using these algorithms, a merged multi-sensor time series of the surface Chl-a concentration in California Current region was created. The merged Oil-a time series (November 1996-December 2011) show a significant (P < 0.01) increasing trend off central California and significant (P < 0.01) decreasing trends in the central North Pacific gyre and off southern Baja California. Although this 15-year time series is too short to separate interannual and multidecadal cycles from climate trends, both of these trends are consistent with the predicted effects of global warming. The expected increase in vertical stratification of the water column and the resulting decreased vertical flux of nutrients would lead to lower Chl-a in the gyre but the increased upwelling-favorable winds leading to stronger upwelling off central California or the increased nitrate content of the upwelled water would lead to higher Chl-a in the upwelling region. The decreased Chl-a off southern Baja California resembles the effect of a decreased influence of strong El Nino events. (c) 2012 Elsevier Ltd. All rights reserved.

Kahru, M, Kudela R, Manzano-Sarabia M, Mitchell BG.  2009.  Trends in primary production in the California Current detected with satellite data. Journal of Geophysical Research-Oceans. 114   10.1029/2008jc004979   AbstractWebsite

Several ocean primary production algorithms using satellite data were evaluated on a large archive of net primary production (NPP) and chlorophyll-a (Chl-a) measurements collected by the California Cooperative Fisheries Investigations program in the California Current. The best algorithm matching in situ data was found by empirically adjusting the Behrenfeld-Falkowski Vertically Generalized Production Model. Satellite-derived time series of NPP were calculated for the California Current area. Significant increase in NPP and Chl-a annual peak levels, i.e., the "bloom magnitude,'' were found along the coasts of the California Current as well as other major eastern boundary currents for the period of modern ocean color data (1997-2007). The reasons for this increase are not clear but are associated with various environmental conditions.

Smith, KL, Ruhl HA, Kaufmann RS, Kahru M.  2008.  Tracing abyssal food supply back to upper-ocean processes over a 17-year time series in the northeast Pacific. Limnology and Oceanography. 53:2655-2667.   10.4319/lo.2008.53.6.2655   AbstractWebsite

Detrital aggregates episodically deposited on the seafloor represent an underestimated food source to deep-sea communities. A 17-yr time-series study was conducted from 1990 to 2006 in the abyssal northeast Pacific (Sta. M, 4100 m in depth) to evaluate the importance of this food source and its temporal relationship to water column and surface ocean processes. Detrital aggregates appeared on the seafloor from June through December, with the highest peaks in 1990, 1994, 2001, and 2002 reaching a maximum density of 23 m(-2) in fall 2001. A total of 15,816 aggregates were measured, most less than 20 cm(2) in area and with a mode of 9 cm(2). Density of detrital aggregates was highly correlated with particulate organic carbon (POC) flux at 600 and 50 m above the bottom (p < 0.001) with no time lag. Export flux of organic carbon from the euphotic zone was significantly correlated with aggregate density, lagged earlier by 1-4 months (p <= 0.001). Zooplankton displacement volume was significantly correlated with POC flux (p =0.023) and with detrital aggregate density (p =0.028) on the seafloor when lagged earlier by <= 1 month. The Bakun upwelling index computed for the region around Sta. M was significantly correlated with detrital aggregate density when lagged earlier by 2-5 months (p < 0.001). A strong correlation exists between surface ocean processes and abyssal food supply, including POC flux and detrital aggregates. This direct coupling through the entire water column must be considered in resolving the marine carbon cycle.

Holm-Hansen, O, Kahru M, Hewes CD, Kawaguchi S, Kameda T, Sushin VA, Krasovski I, Priddle J, Korb R, Hewitt RP, Mitchell BG.  2004.  Temporal and spatial distribution of chlorophyll-a in surface waters of the Scotia Sea as determined by both shipboard measurements and satellite data. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 51:1323-1331.   10.1016/j.dsr2.2004.06.004   AbstractWebsite

Chlorophyll-a (Chl-a) concentrations in surface waters were measured at 137 hydrographic stations occupied by four research vessels participating in the CCAMLR 2000 Survey and the values were compared to estimates from data acquired by the SeaWiFS satellite. The Chl-a concentrations measured on board ship ranged from 0.06 to 14.6 mg m(-3), a range that includes most surface Chl-a concentrations during mid-summer in the Southern Ocean. Owing to persistent cloud cover over much of the Southern Ocean, it was necessary to acquire multi-day composites of satellite data in order to obtain reliable estimates of Chl-a at each of the hydrographic stations. The correlation between the median value for the eight-day composites and the Chl-a concentrations measured on board ship had an R-2 value of 0.82, with the satellite data under-estimating the values obtained on board ship at high Chl-a concentrations and slightly overestimating the shipboard data at Chl-a concentrations of < 0.2 mg m(-3). For Chl-a concentrations of < 1.0 mg m(-3), the ratio of the satellite estimates divided by the shipboard values was 0.89 +/- 0.45 (n = 50). As the mean Chl-a concentration in most pelagic Antarctic waters is close to 0.5 mg m(-3), satellite estimates for Chl-a concentrations in surface waters are thus close to shipboard measurements, and offer the advantage of providing synoptic maps of Chl-a distribution over extensive areas of the Southern Ocean. Satellite Chl-a images for the months preceding (December 1999) and following (February 2000) the CCAMLR 2000 Survey cruises showed that the general pattern of Chl-a concentration in the Scotia Sea and adjoining waters was similar in all three months, but that the phytoplankton biomass was generally lowest in December, reached maximal values in January, and started to decline in February. in contrast, Chl-a concentrations in Drake Passage declined progressively from early December through February. Published by Elsevier Ltd.

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McClatchie, S, Charter R, Watson W, Lo N, Hill K, Manzano-Sarabia M, Goericke R, Collins C, Bjorkstedt E, Schwing FB, Bograd SJ, Kahru M, Mitchell BG, Koslow JA, Ralston S, Field J, Peterson WT, Emmett R, Gomez-Valdes J, Lavaniegos BE, Caxiola-Castro G, Rogers-Bennet L, Gottschalck J, Heureux ML, Xue Y, Munger L, Campbell G, Merkens K, Camacho D, Havron A, Douglas A, Hildebrand J.  2009.  The state of the California Current, Spring 2008-2009: Cold conditions drive regional differences in coastal production. California Cooperative Oceanic Fisheries Investigations Reports. 50:43-68. AbstractWebsite

This report describes the state of the California Current system (CCS) between the springs of 2008 and 2009 based on observations taken along the west coast of North America The dominant forcing on the CCS during this time period were La Nina-type conditions that prevailed from the summer of 2007 through early 2009, transitioning to neutral El Nino-Southern Oscillation conditions in the spring of 2009 The Pacific Decadal Oscillation index was negative during this time period and its values had not returned to normal by the spring of 2009 The general effects on the California Current system were stronger thin normal southward winds and upwelling as well as generally colder than normal SST and shallow nitraclines, however, there were repot-ill differences Off alp California sea surface temperatures did not respond to the La Nina conditions, however, concentrations of chlorophyll a (Chl a) were significantly above normal, probably due to the anomalously high upwelling off Baja during most of the year Off southern California there was no clear evidence of increased primary or secondary production, despite observations that previous La Nina conditions affected mixed layer depth, temperatures, nutrients, and nitracline depths In both central and northern California and Oregon, stronger than normal upwelling increased primary production and prevented potential spawning of sardine north of San Francisco In central California the midwater fish community resembled that of recent cool years, and cover by kelp was much reduced along the coast Off Oregon there was evidence of increased abundance of boreal copepods, although the neritic boreal species did not appear to extend as far south is central California Current predictions are for cooler conditions to change to El Nino conditions by the end of 2009, these are expected to last through the Northern Hemisphere wintet of 2009-10

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McClatchie, S, Goericke R, Koslow JA, Schwing FB, Bograd SJ, Charter R, Lo WWN, Hill K, Gottschalck J, L'Heureux M, Xue Y, Peterson WT, Emmett R, Collins C, Gaxiola-Castro G, Durazo R, Kahru M, Mitchell BG, Hyrenbach KD, Sydeman WJ, Bradley RW, Warzybok P, Bjorkstedt E.  2008.  THE STATE OF THE CALIFORNIA CURRENT, 2007-2008: LA NINA CONDITIONS AND THEIR EFFECTS ON THE ECOSYSTEM. California Cooperative Oceanic Fisheries Investigations Reports. 49:39-76. AbstractWebsite

The state of the California Current system (CCS) between Oregon and Baja California is summarized in this report, covering spring of 2007 to winter/spring 2008. The 2006-07 period began with moderate El Nino conditions which decayed rapidly in early 2007. By summer 2007, a moderate-to-strong La Nina had developed. The North Pacific sea surface temperature (SST) anomalies displayed a negative pattern of Pacific Decadal Oscillation with below-normal SSTs in the California Current and Gulf of Alaska consistent with this pattern. The region experienced anomalously strong southward coastal winds, leading to positive anomalies of the West Coast upwelling index, in Strong contrast with 2005. The 2007 upwelling season also began early (in contrast to delayed onset in 2005 and 2006) and remained unseasonably strong through May. The cumulative upwelling for the 2007 season was greater than normal in the southern portion of the California Current system. Despite the La Nina conditions, nitrate and chlorophyll concentrations off Oregon were about average in 2007. On the other hand, copepod biomass rebounded strongly in 2006 after the exceptionally low biomass in 2005, and copepod species richness in 2006 was low, also indicating transport Of sub-arctic water into the northern California Current in 2006-07, which is relatively productive but low in diversity. Anomalously high salinities at 200 in depth were also observed during CalCOFI and IMECOCAL cruises off Southern and Baja California. In the CalCOFI area, where there has been a general trend toward a deepening mixed layer, the mixed layer responded to this year's La Nina conditions by shoaling. Nitrate (but not silicate and phosphate) concentrations in the mixed layer were anomalously high, but chlorophyll concentrations were about average, except for spring 2007, which was one of the lowest values on record. Spring chlorophyll a concentrations are notably variable during La Ninas. In the northern California Current, forage fish and predatory fish abundance remained low in 2007. In the southern California Current, Pacific sardine (Sardinops sagax) larval abundance was relatively high and distributed in relation to the inner edge of the California Current and the edge of an eddy. Northern anchovy (Engraulis mordax) larvae were relatively low in abundance, apparently related to a large downwelling feature. Reproductive success of all six seabirds monitored on Farallon Island was recovering slowly this year, following the previous two disastrous seasons. However, cluster analysis indicated that reproductive success is still relatively low. The cold-water planktivorous auklets (Ptychoramphus aleuticus) continued to be found at high densities in Southern waters. Overall, the transition in 2007 to La Nina conditions appeared to contribute to average to above average productivity in the California Current, but the physical, chemical, and biological (phytoplankton, zooplankton, Fish, and seabird) indices of productivity were far from consistent.

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Leising, AW, Schroeder ID, Bograd SJ, Bjorkstedt EP, Field J, Sakuma K, Abell J, Robertson RR, Tyburczy J, Peterson WT, Brodeur R, Barcelo C, Auth TD, Daly EA, Campbell GS, Hildebrand JA, Suryan RM, Gladics AJ, Horton CA, Kahru M, Manzano-Sarabia M, McClatchie S, Weber ED, Watson W, Santora JA, Sydeman WJ, Melin SR, DeLong RL, Largier J, Kim SY, Chavez FP, Golightly RT, Schneider SR, Warzybok P, Bradley R, Jahncke J, Fisher J, Peterson J.  2014.  State of the California Current 2013-14: El Nino looming. California Cooperative Oceanic Fisheries Investigations Reports. 55:51-87. AbstractWebsite

In 2013, the California current was dominated by strong coastal upwelling and high productivity. Indices of total cumulative upwelling for particular coastal locations reached some of the highest values on record. Chlorophyll a levels were high throughout spring and summer. Catches of upwelling-related fish species were also high. After a moderate drop in upwelling during fall 2013, the California current system underwent a major change in phase. Three major basin-scale indicators, the PDO, the NPGO, and the ENSO-MEI, all changed phase at some point during the winter of 2013/14. The PDO changed to positive values, indicative of warmer waters in the North Pacific; the NPGO to negative values, indicative of lower productivity along the coast; and the MEI to positive values, indicative of an oncoming El Nino. Whereas the majority of the California Current system appears to have transitioned to an El Nino state by August 2014, based on decreases in upwelling and chlorophyll a concentration, and increases in SST, there still remained pockets of moderate upwelling, cold water, and high chlorophyll a biomass at various central coast locations, unlike patterns seen during the more major El Ninos (e.g., the 97-98 event). Catches of rockfish, market squid, euphausiids, and juvenile sanddab remained high along the central coast, whereas catches of sardine and anchovy were low throughout the CCS. 2014 appears to be heading towards a moderate El Nino state, with some remaining patchy regions of upwelling-driven productivity along the coast. Superimposed on this pattern, three major regions have experienced possibly non-El Nino-related warming since winter: the Bering Sea, the Gulf of Alaska, and offshore of southern California. It is unclear how this warming may interact with the predicted El Nino, but the result will likely be reduced growth or reproduction for many key fisheries species.

Kahru, M, Mitchell BG.  1998.  Spectral reflectance and absorption of a massive red tide off southern California. Journal of Geophysical Research-Oceans. 103:21601-21609.   10.1029/98jc01945   AbstractWebsite

Spectral reflectance and absorption of a massive Lingulodinium (Gonyaulax) polyedra red tide in March 1995 off southern California are compared to a "baseline" of biooptical measurements from the California Cooperative Oceanic Fisheries Investigations. The red tide was characterized by increased absorption and therefore reduced remote sensing reflectance (R(rs)) in the 340-400 nm spectral range. The increased ultraviolet absorption was probably caused by mycosporine-like amino acids in the particulate fraction as well as increased absorption by dissolved organic matter. The chlorophyll a (chl a) specific particulate absorption of the L. polyedra bloom in the visible spectral range remained relatively constant for the chi a range 1-150 mg m(-3) indicating accumulation of cells with similar optical characteristics. The difference in the R(rs) versus chl a relationship of the red tide and "normal" California Current phytoplankton diminished with increasing wavelength from 340 nm and disappeared at 412 nm. Ratios of R(rs) at 340 nm (or 380 nm) and 412 nm (or 443 nm) provided differentiation of the red tide starting at chi a concentration of 1-2 mg m(-3). The forthcoming Japanese Global Imager (GLI) satellite sensor has, among others, the 380 nm band. If the signal to noise ratio and atmospheric correction for the 380 nm band are sufficient to retrieve the dynamic range of the water leaving radiance, then it might be possible to differentiate red tides from other phytoplankton bloods with the algorithm described here.

Kahru, M, Leppanen JM, Nommann S, Passow U, Postel L, Schulz S.  1990.  Spatio-temporal Mosaic of the Phytoplankton Spring Bloom in the Open Baltic Sea in 1986. Marine Ecology-Progress Series. 66:301-309.   10.3354/meps066301   AbstractWebsite

The spatio-temporal variability of the phytoplankton spring bloom in the open Baltic Sea during an international joint study (PEX'86) is described in relation to the physical and chemical environment. Simultaneous participation of 14 research vessels enabled highly synoptic sampling of the study area of 20 X 40 n mile. Onset of the bloom was controlled by mesoscale hydrodynamic features - a pair of cyclonic and anticyclonic eddies; water masses with different salinities. The eddies supported an exceptionally high amount of chlorophyll (> 700 mg m-' in the cyclonic eddy). The temporal development of the bloom was different in each of the mesoscale features. Chlorophyll levels in the eddies started to decrease due to segmentation before formation of the thermocline and the depletion of inorganic nutrients. Variability of the chlorophyll distribution was highest just before the peak in the spatially averaged concentrahon was attained.

Kahru, M, Aitsam A, Elken J.  1982.  Spatio-Temporal Dynamics of Chlorophyll in the Open Baltic Sea. Journal of Plankton Research. 4:779-790.   10.1093/plankt/4.4.779   AbstractWebsite

Five charts of the chlorophyll and hydrographic fields by vertical profiles of in situ fluorescence and CTD were made on a stationary grid of 20 × 25 nautical miles with a 5-mile spacing in the open Baltic Sea. Both chlorophyll levels and variability were maximal close to the spring bloom. High chlorophyll levels in summer are sustained by recurrent nutrient injections from the deep saline layer. Two of the surveys showed close coupling between the coarse-scale (∼10 km) chlorophyll distribution and the hydrographic structure determining the intensity of nutrient transfer. Vigorous advection, stirring and current shear, associated with a strong mesoscale eddy, probably dominated the chlorophyll pattern on three surveys. The upward velocities in the cyclonic eddy resulted in accumulation of phytoplankton in the aphotic zone. Intensive heat input from the surface caused a sudden sinking of the phytoplankton and the formation of a pronounced sub-surface chlorophyll maximum.

Kahru, M, Di Lorenzo E, Manzano-Sarabia M, Mitchell BG.  2012.  Spatial and temporal statistics of sea surface temperature and chlorophyll fronts in the California Current. Journal of Plankton Research. 34:749-760.   10.1093/plankt/fbs010   AbstractWebsite

The statistics of sea-surface fronts detected with the automated histogram method were studied in the California Current using sea-surface temperature (SST) and chlorophyll-a concentration (Chl) images from various satellite sensors. Daily maps of fronts were averaged into monthly composites of front frequency (FF) spanning 29 years (19812009) for SST and 14 years (19972010) for Chl. The large-scale distributions of frontal frequency of both SST (FFsst) and of Chl (FFchl) had a 500700 km wide band of elevated values (47) along the coast that roughly coincided with the area of increased mesoscale eddy activity. FFsst and FFchl were positively correlated at monthly and seasonal frequencies, but the year-to-year variations were not significantly correlated. The long-period (1 year and longer) variability in FFsst is influenced by the large-scale SST gradient, while at shorter timescales the influence of the Coastal Upwelling Index is evident. In contrast with FFsst, FFchl variability is less related to the coherent large-scale forcing and has stronger sensitivity to local forcings in individual areas. Decadal-scale increasing trends in the frequency of both SST and Chl fronts were detected in the Ensenada Front area (general area of the A-Front study) and corresponded to, respectively, trends towards colder SST and increasing chlorophyll-a concentration.

Kahru, M, Leeben A.  1991.  Size Structure of Planktonic Particles in Relation to Hydrographic Structure in the Skagerrak. Marine Ecology-Progress Series. 76:159-166.   10.3354/meps076159   AbstractWebsite

Horizontal distributions of the particle size spectrum and chlorophyll a concentration in near-surface water in relation to hydrographic structure were studied along a transect across the Skagerrak as part of the international SKAGEX-I exercise in May-June, 1990. Results are compared with those of Kiorboe et al. (Mar. Ecol. Prog. Ser. 59: 19-32, 1990) who explained changes in the plankton size structure and the food web as resulting from differences in water column stratification. It was confirmed that the particle size fraction corresponding to diatoms was most abundant along the Skagerrak periphery where stronger vertical mixing is assumed to take place. The smallest (1 to 4-mu-m), chlorophyll-correlated particulate fraction was most abundant in the central Skagerrak with the dome-like near-surface stratification and the presumably reduced vertical mixing. However, distributions along the transect changed considerably over 1 to 3 d due to advection. In addition to the basin-wide changes, correlations of the different particle size fractions to chlorophyll concentration changed over much shorter length scales, often being in opposite phase to each other. An equivalent spherical diameter of 4-mu-m was the size at which the most significant shift in the horizontal distributions occurred.

Kahru, M, Mitchell BG.  2001.  Seasonal and nonseasonal variability of satellite-derived chlorophyll and colored dissolved organic matter concentration in the California Current. Journal of Geophysical Research-Oceans. 106:2517-2529.   10.1029/1999jc000094   AbstractWebsite

Time series of surface chlorophyll a concentration (Chl) and colored dissolved organic matter (CDOM) derived from the Ocean Color and Temperature Sensor and Sea-Viewing Wide Field-of-View Sensor were evaluated for the California Current area using regional algorithms. Satellite data composited for 8-day periods provide the ability to describe large-scale changes in surface parameters. These changes are difficult to detect based on in situ observations alone that suffer from undersampling the large temporal and spatial variability, especially in Chi. We detected no significant bias in satellite Chi estimates compared with ship-based measurements. The variability in CDOM concentration was significantly smaller than that in Chi, both spatially and temporally. While being subject to large interannual and short-term variations, offshore waters (100-1000 km from the shore) have an annual cycle of Chi and CDOM with a maximum in winter-spring (December-March) and a minimum in late summer. For inshore waters the maximum is more likely in spring (April-May). We detect significant increase in both Chi and CDOM off central and southern California during the La Nina year of 1999. The trend of increasing Chi and CDOM from October 1996 to June 2000 is statistically significant in many areas.

Kahru, M, Fiedler PC, Gille ST, Manzano M, Mitchell BG.  2007.  Sea level anomalies control phytoplankton biomass in the Costa Rica Dome area. Geophysical Research Letters. 34   10.1029/2007gl031631   AbstractWebsite

Satellite data show that chlorophyll-a concentration (Chl-a) in the northeastern tropical Pacific is well correlated with sea level anomaly (SLA). This correlation spans a wide spectrum of scales from large-scale phenomena like ENSO to mesoscale cyclonic and anticyclonic eddies. Negative SLA (e. g. during La Ni (n) over tildea events and in cyclonic eddies) is associated with the lifting of isopycnals in the nutricline and increased Chl-a due to enhanced phytoplankton growth, while positive SLA (e. g. during El Ni (n) over tildeo events and in anticyclonic eddies) is associated with a deeper nutricline and reduced Chl-a due to decreased phytoplankton growth. The coupling between SLA and Chl-a anomaly in the Costa Rica Dome (CRD) area is tighter than has previously been recorded anywhere in the world ocean. 70% of the interannual variations in Chl-a anomaly in the CRD area is explained by a combination of the positive and negative effects of SLA.

Kahru, M, Savchuk OP, Elmgren R.  2007.  Satellite measurements of cyanobacterial bloom frequency in the Baltic Sea: interannual and spatial variability. Marine Ecology-Progress Series. 343:15-23.   10.3354/meps06943   AbstractWebsite

Owing to the potentially harmful character of nitrogen-fixing filamentous cyanobacterial blooms in the Baltic Sea, a capacity to predict their occurrence is of interest. We quantified the surface accumulations of cyanobacteria, mainly Nodularia spumigena, using ocean colour satellite data. The spatial and temporal frequency of the accumulations was mapped with an automated detection algorithm, using their high reflectance in the 670 or 555 nm bands. Coastal Zone Color Scanner (CZCS) data were used for a first period (1979 to 1984), and combined SeaWiFS and MODIS-Aqua data for a second period (1998 to 2006). The frequency of cyanobacterial accumulations (FCA) for each 1 km(2) pixel was calculated as the ratio of scenes with detected cyanobacteria to the number of valid cloud-free scenes in July-August. FCA varied greatly among years and the basins of the Baltic Sea. Mean FCA was 39% higher in the second period, but the increase was not statistically significant. The mean FCA for the whole Baltic was positively correlated with the residual phosphate (RP) concentration after the spring bloom in May-June. RP was a useful predictor of FCA in the coming summer, but could not predict the spatial pattern of the bloom. This was better explained by prevailing winds, with high FCA in the southwestern Baltic after northerly winds in July. It seems that, at the time of writing, useful FCA predictions can be made after the spring bloom, but only for the whole Baltic.

Kahru, M, Horstmann U, Rud O.  1994.  Satellite detection of increased cyanobacteria blooms in the Baltic Sea: Natural fluctuation or ecosystem change? Ambio. Stockholm. 23:469-472. AbstractWebsite

Using data from the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA series of satellites, an increase in the area covered by cyanobacteria blooms in the Baltic Sea was detected. The time series of satellite data covers a period of 12 years from 1982 to 1993. The total area covered by surface-floating cyanobacteria (blue-green algae) has increased in the 1990s, reaching over 62 000 km in 1992. From 1992, visible accumulations appeared for the first time in the Gulf of Riga and reappeared, in the western Gulf of Finland, after being absent from 1984. Conspicuous surface blooms were also present in the early 1980s, coincident with a period of sunny and calm summers. However, when the influence of variable sunshine duration is taken into account, the increase in 1991-1993 is still distinct, indicating significant changes in the Baltic environment. The causal factors for the increased cyanobacteria blooms are still not clear.